JPH11315328A - Manufacture of hot rolled high tensile strength steel plate having superior workability and excellent in shape freezability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of manufacturing a high tensile strength steel plate for working, which is excellent in shape freezability and capable of extending the range of application of a high tensile strength steel plate even to the part conventionally difficult because of the problem of dimensional accuracy and contributing, e.g. to weight reduction of an automobile. SOLUTION: When hot rolling a steel which has a composition containing, by weight ratio, 0.05-0.2% C, 0.3-2.5% Si, 0.5-2.5% Mn, <=0.15% P, and 0.01-2.5% Al and also containing, if necessary, <=0.05% Ti, <=0.05% Nb, and <=0.005% B independently or in combination, finishing is done at >=650 deg.C and also rolling of >=50% total draft is performed at a temperature between the Ar3 transformation point and 650 deg.C while applying lubrication so that a coefficient of friction becomes <=0.2. The resultant steel plate is temporarily coiled at 800 to 650 deg.C, held for 10 sec to 10 min, uncoiled, cooled at >=20 deg.C/sec cooling rate, and coiled again at 500 to 300 deg.C. By this method, the hot rolled high tensile strength steel plate having superior workability and excellent in shape freezability can be manufactured.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a hot-rolled high-strength steel sheet having good formability and good workability, which is used for underbody parts for automobiles and the like.

[0002]

2. Description of the Related Art Good workability hot-rolled high-tensile steel sheets are also referred to as retained austenite steel sheets, and have a structure in which retained austenite is scattered in a structure mainly composed of ferrite and bainite.
The feature of this steel sheet is that while maintaining good ductility with a ferrite structure, strength is given to bainite, and residual austenite is transformed into martensite with forming and strengthens the place where constriction is likely to occur, thereby suppressing fracture. And exhibit high ductility. This mechanism shows much better workability than conventional high-strength steel, and its application to difficult-to-machine parts has been promoted.

[0003] However, this widespread application has its limitations. One of the main factors is the problem of shape freezing. Retained austenitic steel generally has a good balance between strength and ductility and can strengthen the wrinkle suppressing force, so the formability limit is relatively wide compared to other high-strength steels, but it is not necessarily excellent in shape freezing property. I can not say.

[0004] The feature of the composition of this steel sheet is that C and Mn are added in order to obtain high tensile strength. However, these elements are austenitic stable elements and suppress the formation of ferrite. This is a component system in which ferrite is sufficiently precipitated by adding Si, Al, P and the like (for example, Japanese Patent Application Laid-Open No. 61-157625). In particular, Al and Si have the effect of delaying the precipitation of cementite, promote the concentration of solid solution C in austenite, and have the effect of making it easier to form retained austenite.

[0005] Further, the feature of the manufacturing method is that, in order to sufficiently precipitate the ferrite, the finish rolling is performed at a temperature not lower than the Ar ₃ transformation point, and then, in order to promote the precipitation of the ferrite, in a temperature range where the ferrite is easily precipitated. Slowly cooling, then quenching so that pearlite does not precipitate, winding at bainite formation temperature, enriching C in austenite with progression of bainite, stabilizing austenite, leaving austenite stable at room temperature It has a cooling pattern (for example, JP-A-2-217425). As described above, since hot rolling itself is performed at the Ar ₃ transformation point or higher, the texture of the sheet thickness center layer is randomized by transformation, and the ratio of {100} / {111} is often around 1. That is, there has been no development focusing on the texture.

[0006]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method for producing a hot-rolled high-tensile steel sheet having good formability and good workability.

[0007]

The present inventors have studied in detail the relationship between the shape freezing property and the mechanical properties, and have found that the shape freezing property is greatly affected by the work hardening characteristics during molding. The present invention is based on these findings, and the gist thereof is as follows. (1) By weight ratio, C: 0.05 to 0.2%, Si: 0.3 to 2.5%, Mn: 0.5
~ 2.5%, P: 0.15% or less, Al: 0.01 ~ 2.5%, when hot rolling steel consisting of iron and unavoidable impurities, 650 ℃
With finish above, Ar ₃ or less transformation point, the total reduction ratio at 650 ° C. or higher temperatures performs rolling over 50%, 800 ° C. or less,
Winding once at a temperature of 650 ° C or more, holding for 10 seconds or more and 10 minutes or less, rewinding, cooling at a cooling speed of 20 ° C / sec or more, and winding again at a temperature of 500 ° C or less and 300 ° C or more A method for producing a hot-rolled high-tensile steel sheet having good formability and excellent workability.

(2) Ti: 0.001 to 0.05% by weight,
Nb: 0.001-0.05% and B: 0.0001-0.005%
(1) characterized in that it contains at least one species or two or more species.
The method for producing a hot-rolled high-tensile steel sheet having excellent shape freezing property according to item 1. (3) The total rolling reduction is below the Ar ₃ transformation point and above 650 ° C.
When rolling over 50%, lubrication is applied and the coefficient of friction is 0.2
The method for producing a hot-rolled high-tensile steel sheet having good formability and excellent workability according to the above (1) or (2), wherein the rolling is performed below.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail. First, conditions for limiting the components of the steel sheet of the present invention will be described.

C is limited to not less than 0.05% and not more than 0.2%. This is because when C is less than 0.05%, the strength as a high-tensile steel sheet is not sufficient, and when C exceeds 0.2%, the weldability deteriorates.

The content of Si is limited to 0.3% or more and 2.5% or less.
This is because when Si is less than 0.3%, retained austenite and ferrite are hardly generated, and when it exceeds 2.5%, workability is deteriorated.

Mn is limited to 0.5% or more and 2.5% or less. This means that if Mn is 0.5% or more, strength can be secured, and 2.
If it exceeds 5%, workability is deteriorated. P promotes the formation of ferrite similarly to Si, but excessive addition deteriorates the workability, so the upper limit was made 0.15%. Al is necessary at the time of deoxidation, and the lower limit at that time is 0.01%. Al promotes the formation of ferrite with an effect similar to that of Si, but excessive addition deteriorates workability, so the upper limit was made 2.5%.

[0013] Ti, Nb, and B to be added selectively improve the material by controlling the structure through mechanisms such as precipitation strengthening, fine grain strengthening, and transformation strengthening. Therefore, moderate addition is preferable. Are degraded, so the upper limit is set to 0.
It was limited to 05%, 0.05%, and 0.005%. Ti,
Nb and B are the lower limits at which the respective effects can be obtained,
Ti: 0.001%, Nb: 0.001%, B: 0.0
001%.

Next, limitations on the manufacturing process conditions of the present invention will be described. Under hot rolling conditions, the lower limit of the finishing temperature is 65
The reason why the temperature is set to 0 ° C. is that when rolling is performed at a lower temperature, recrystallization does not sufficiently occur during subsequent winding, resulting in deterioration in workability. The Ar ₃ transformation point or less, the total draft is limited to 50% or more at a temperature of 650 ° C. or more, because the shape freezing property is significantly improved by satisfying this condition, which is obtained under the above conditions. It is presumed that a texture having excellent shape freezing properties is formed.

In addition, lubrication is performed when hot rolling with a total reduction of 50% or more at a temperature of 650 ° C. or more at an Ar ₃ transformation point or less, and the friction coefficient between the roll and the steel sheet is reduced to 0.2 or less. It was found that the freezing property was further improved. The reason is presumed to be that the texture in the surface layer portion was made closer to the central texture by reducing the shear deformation, so that the texture advantageous for shape freezing spread throughout the plate thickness.

Next, limitations on the winding conditions of the steel sheet will be described. A major feature of the present invention is that the winding process is performed twice. In the first winding, recrystallization treatment is actively performed, and in the second winding, quenching is performed at a low temperature after quenching to suppress carbide precipitation due to pearlite transformation and the like. This is to leave austenite stable even at room temperature.

The reason why the first winding treatment is limited to a temperature of 800 ° C. or less, 650 ° C. or more, 10 seconds or more and 10 minutes or less is that ferrite is not sufficiently generated at a temperature of more than 800 ° C.
The reason why the lower limit temperature is set to 650 ° C. is that the generation of pearlite becomes evident at a temperature lower than this, and thereafter, even if the low-temperature winding is performed in the second winding, no residual austenite is generated. This is because characteristics cannot be obtained. The holding time of the first winding is set to 10 seconds or more because the possibility that the processed ferrite will not be sufficiently recrystallized in a shorter time is increased. The reason why the upper limit is set for the winding holding time is that a long holding time deteriorates productivity.

The reason why the cooling rate from the start of the unwinding of the coil to the second winding is limited to 20 ° C./sec or more is to suppress generation of pearlite and to generate appropriate amounts of bainite and residual austenite. . In order to suppress the formation of pearlite and promote the bainite transformation, the second winding temperature needs to be 500 ° C. or less. In addition, since the formation of martensite must be suppressed in order to form retained austenite, the lower limit of the second winding temperature is set to 300.
℃. Such winding conditions are wound by a coiler relatively close to the finishing mill, and then the ROT (Ru
It is realized by rewinding to the n-out Table) and winding it again with a conventional coiler.

The effect of the present invention can be obtained even when the high-tensile steel sheet according to the present invention is used as a surface-treated original sheet.

[0020]

The present invention will be described below with reference to examples and comparative examples.

In Examples and Comparative Examples, steels having the component compositions shown in Table 1 were used. Steel types A to C and EG are steels of the present invention, and D, H and I are comparative steels. The Ar ₃ temperature is a measurement result when cooled by a Formaster at 5 ° C./sec. Hot rolled
The product of total elongation and tensile strength of tensile test of JIS No. 5 test piece cut out in the rolling direction of a product sheet, which is an index of winding conditions and formability of high tensile steel sheet, and after 90 degree bending which is an index of shape freezing property Table 2 shows a value obtained by subtracting the opening angle of the sample by 90.

In Table 2, the first winding time is defined as
It is defined as the time from being wound on the coil to being unwound again and leaving the coil. Other manufacturing conditions are as follows: slab heating temperature is 950 ° C to 1250 ° C, hot rolled sheet thickness is 1.6mm
And The coefficient of friction was determined by calculation based on rolling theory from data such as advanced ratio, rolling load, and torque.

[0023]

[Table 1]

[0024]

[Table 2]

Experiment Nos. 2 and 3 satisfying the scope of the present invention
5, 7, 9, 10, 12, 14, 17, 19, 22, 2,
The materials Nos. 4 and 26 have not only excellent strength-ductility balance, but also a small springback angle and excellent shape freezing properties. On the other hand, Experiment Nos. 1, 16, 18, 21, 23, produced by a single winding method in a normal γ-range hot rolling,
The material No. 25 has significantly larger springback than the steel of the present invention of the same steel type.

The sample of Experiment No. 4 in which the total reduction in the temperature range of Ar ₃ or less and 650 ° C. or more was as low as 35% had a large springback amount.

Experiment No. 6 where the first winding temperature was low
In the sample No., pearlite was formed, and the strength-ductility balance was deteriorated.

The material of Experiment No. 8 in which the holding time during the first winding was short, not only had a poor strength-ductility balance, but also had a large springback angle.

From the start of rewinding after the first winding, 2
The material of Experiment No. 11 in which the average cooling rate until the second winding was slow had a poor strength-ductility balance.

The samples of Experiment Nos. 13 and 15 in which the second winding temperature was out of the range of the present invention did not show an excellent strength-ductility balance. In particular, the sample of Experiment No. 15
Since martensite was formed, the strength increased and the amount of springback increased.

In Test Nos. 20, 27 and 28 using steels whose component ranges deviated from those of the steel of the present invention, no excellent strength-ductility balance was obtained.

[0032]

According to the method of the present invention, it is possible to provide a high-strength steel sheet for processing excellent in shape freezing property, and to use a high-tensile steel sheet even in a portion difficult to apply due to a problem of dimensional accuracy, thereby reducing the weight of an automobile. Etc. can be contributed.

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Matsuo Usuda 20-1 Shintomi, Futtsu City Nippon Steel Corporation Technology Development Division

Claims (3)

[Claims] (1) C: 0.05 to 0.2%, Si: 0.3 to 2.5% by weight,
Mn: 0.5 to 2.5%, P: 0.15% or less, Al: 0.01 to 2.5%,
When hot rolling steel consisting of the balance iron and unavoidable impurities,
Finishing at 650 ° C or higher, rolling at a temperature of 650 ° C or higher with a total draft of 50% or higher at an Ar ₃ transformation point or lower, 800 ° C
Below, take up once at a temperature of 650 ° C or more, 10 seconds or more, 10
Good workability with excellent shape freezing characteristics, characterized by rewinding after holding for less than a minute, cooling at a cooling speed of 20 ° C / sec or more, and winding again at a temperature of 500 ° C or less and 300 ° C or more. Manufacturing method of hot-rolled high-tensile steel sheet. 2. Ti: 0.001-0.05%, Nb: 0.001-0.00% by weight
The method for producing a hot-rolled high-strength steel sheet excellent in shape freezing property according to claim 1, comprising one or more of 0.05% and B: 0.0001 to 0.005%. _3. Rolling is performed at a temperature not higher than the Ar ₃ transformation point and at a temperature not lower than 650 ° C. and a total reduction ratio of not less than 50%.
Or the method for producing a hot-rolled high-strength steel sheet having excellent workability as described in 2 above.